What's the difference between 'collision' and 'conflict'? In SQL language, should I say 'PK collision' or 'PK conflict'?
A collision is usually taken to mean a physical impact: two cars which crash into each other would be a collision.
A conflict is normally between two or more things which disagree in some way, although its meaning can be extended across both tangible and intangible notions. For instance, you can have a conflict of opinions, where someone disagrees with the other, but you can also have a physical conflict (a fight, or a war).
Conflict, used in its physical sense, could be synonymous with collision.
In your case, you should say a Primary Key (PK) Conflict, as — and I'm assuming it's a duplicate here — the primary keys disagree with each other because they are the same.
They don't physically bump into each other.
And for the second question you should call it as Primary Key Conflict.